Saturday, July 30, 2011

Power lines from Landsat imagery

Hydropower is the big deal in Laos at the moment and therefore power lines are required. Some weeks ago on an offroad ride we crossed the relatively new Nam Ngum 2power transmission line. Realizing the long straight and wide forest aisle, I knew that it must be possible to identify the line on Landsat imagery to map it on OpenStreetMap. Landsat imagery is in the public domain and thus suitable for mapping.

I downloaded the Landsat imagery from January and February 2011 for this region already earlier. Therefore I could start mapping the power line immediately, starting from the point where the power line crossed our way and following the forest aisle on Landsat. It's probably the first mapped power line on OpenStreetMap in Laos.

Nam Ngum 2 dam and power lines on Landsat

There is the Nam Ngum 2 reservoir and its dam visible in the middle at the top border. In the lower left corner is the older Nam Ngum 1 reservoir. The power line going south is clearly identifiable in the middle section. Furthermore two access roads to the dam are identifiable, one on the north shore of Nam Ngum 1 and one leading east from the dam. Just downstream of the new dam there is the confluence of Nam Bak.

Just for the visual appearance I tweaked the image to get rid of the stripes using GDAL, GIMP and OSSIM. OSSIM is as fas as I know still the only FOSS that implements histogram stretch based on the standard deviation.